The problem addressed by this invention is that of optical detection devices in which an input beam will be divided into two or more output beams of different intensity, the intensities being selected so that when the first beam saturates the detector, the second beam will have a lower intensity that can be measured by the detector. Such an approach is necessary in samples with rapid temporal variations that cause signal levels to vary widely and randomly with time.
The use of a glass plate or wedge to derive two or more beams from an input beam is well known in the art. U.S. Pat. No. 4,362,361 entitled "Collimated Beam Manifold with the Number of Output Beams Variable in a given Output Angle", illustrates the approach using a glass plate to produce a number of parallel output beams having a specified intensity ratio. This device accepts as input a collimated beam and produces a number of collimated beams as output.
In the particular case where the optical radiation has to come to a focal point at the surface of a multichannel detector, with each channel representing a different frequency range of the radiation, the foregoing approach will not work because the several output beams must all focus on the detector surface or else the spectral response of the detector will be different for each output beam. A straightforward approach using the prior art would be to position a different focal length lens in front of each output beam to provide a focus at the right position. In the real world, however, this approach would not work because commercially available multichannel detectors are quite small.